This disclosure relates to crack free multilayered devices, methods of manufacture thereof and articles comprising the same. In particular, this disclosure relates to crack free group III nitrides disposed on a substrate.
Epitaxial growth of gallium nitride (GaN) occurs on gallium nitride substrates cut from bulk gallium nitride single crystals or on sapphire substrates. The sapphire substrates may be oriented in c-direction, a-direction, r-direction or m-direction; on axis or of-cut. Bulk crystal growth of gallium nitride, however uses extremely high pressures to maintain the nitrogen content in the crystal, rendering bulk growth extremely difficult. For this reason, the high volume production of large size, bulk gallium nitride is improbable in the near future and the search for alternative substrates continues.
Two of the main factors associated with substrate choice are cost and resulting gallium nitride epilayer quality. Silicon is increasingly being used as a substrate for gallium nitride deposition because silicon substrates are available at comparatively low cost, high quality, large area, and in large quantities thus presenting many manufacturing advantages over other available substrates for gallium nitride, such as sapphire and silicon carbide (SiC). The disadvantages of silicon as a substrate for gallium nitride heteroepitaxy include an a-plane +20.5% misfit which led to the conclusion that growth of gallium nitride directly on silicon was unfeasible.
Moreover, the thermal expansion misfit between gallium nitride (5.6×10−6 K−1) and silicon (6.2×10−6 K−1) can lead to cracking upon cooling in films grown at high temperature, and, at elevated temperature, melt-back etching between gallium nitride and the silicon substrate during the initial stages of growth or at stress is known to induce cracks that form in gallium nitride films during gallium nitride deposition.
To circumvent these thermal expansion mismatches, a thin aluminum nitride buffer layer is used to absorb the lattice mismatch between the gallium nitride film and the silicon substrate. The subsequent deposition of gallium nitride introduces significant strain into the structure due the large lattice mismatch along with the resultant high density of defects that introduce additional tensile stress into the film. This tensile stress is exacerbated during cool down from growth temperature with macro-crack formation customary for gallium nitride films thicker than 1 micrometer.
To overcome gallium nitride cracking problems, different techniques have been used including multiple aluminum nitride interlayers, aluminum gallium nitride graded layers, patterned silicon, and in situ silicon nitride masking (non-uniform deposition). These methods were reported to provide some decrease in bowing and cracking, but no method successfully produced crack-free thick (e.g. >10 micrometer) gallium nitride films likely because there still remains excessive tensile stress, as well as strong cohesion between gallium nitride (or aluminum nitride buffer layer) and silicon. Although ˜7 micrometer thick crack-free gallium nitride on silicon has been reported by incorporating multiple aluminum nitride interlayers, the maximum thickness of a commercially available crack-free gallium nitride layer on silicon is about 1 micrometer.
Cracks can be generated during growth or cooling due to the excess tensile stress caused by large lattice and thermal expansion differences. It has been observed that the cracks penetrate through the silicon substrate and separation occurs inside the silicon substrate. The strong cohesion between gallium nitride and silicon (or aluminum nitride and silicon in a system comprising gallium nitride/aluminum gallium nitride/aluminum nitride/silicon) as well as the brittleness of silicon, are responsible for cracking to take place in the interior of the silicon wafer. The bond strength of Si—Si is 7 electron Volts (eV), which is lower than the gallium nitride (8.9 eV) or Al—N (11.5 eV) and Si—N (10.5 eV). The bond strength of Si—Si is the weakest. The nano-indentation hardnesses of the GaN, MN, and Si are 20, 18 and 14 gigapascals (GPa) respectively. Therefore, the cracking penetration to the silicon substrate was expected. This brittleness of silicon added with the large tensile stress created by the lattice mismatch and thermal expansion differences makes the growth of crack-free gallium nitride on silicon even more challenging.